Plated steel sheet for hot press forming having excellent weldability and corrosion resistance, forming member, and manufacturing method thereof

ABSTRACT

The present invention relates to a plated steel sheet for hot press forming, a hot-press forming member manufactured using the same, and a manufacturing method thereof. The plated steel sheet comprises: a base steel sheet and a composite plated layer that is formed on at least one surface of the base steel sheet, and has a Mn-based plated layer and an Al-based plated layer alternately formed therein, an Al-based plated layer being formed on the uppermost layer thereof, wherein the composite plated layer has a total thickness of 5 to 30 μm, and in this case, the Mn-based plated layer accounts for 5 to 60% of the total thickness.

RELATED APPLICATIONS

This application is a divisional patent application of U.S. patentapplication Ser. No. 15/102,833, filed on Jun. 8, 2016, which is theU.S. National Phase under 35 U.S.C. § 371 of International ApplicationNo. PCT/KR2014/012273, filed on Dec. 12, 2014, which in turn claims thebenefit of Korean Patent Application No. 10-2013-0160776 filed on Dec.20, 2013, the disclosure of which applications are incorporated byreference herein.

TECHNICAL FIELD

The present disclosure relates to a plated steel sheet for hot pressforming (HPF) and a formed member, and more particularly, to a platedsteel sheet having high weldability and corrosion resistance formanufacturing automotive components by HPF, a member formed using theplated steel sheet, and methods of manufacturing the plated steel sheetand the formed member.

BACKGROUND ART

Recently, the application of high-strength steels to automobiles hasincreased for the purpose of weight reduction. However, high-strengthsteels are easily fractured when processed at room temperature. Inaddition, since high-strength steels undergo spring back during formingprocesses, it is difficult to precisely determine the dimensionhigh-strength steels so as to manufacture complicated parts. Due tothese reasons, hot press forming (HPF) has become favored as a methodfor shaping high-strength steels.

HPF is a method of forming steel sheets into complicated shapes at hightemperatures at which steel sheets become soft and ductile. In detail,HPF is performed by heating a steel sheet to an austenite temperaturerange or higher, pressing the steel sheet, and rapidly cooling the steelsheet simultaneously with the pressing so as to cause the steel sheet toundergo transformation into martensite. In this manner, precisehigh-strength products may be manufactured by HPF.

However, if steel is heated to a high temperature, surface corrosion ordecarbonization may occur. Thus, steel plated with a Zn-based orAl-based material is widely used in an HPF process. In particular,galvanized steel sheets having Zn-based plating layers are highlyresistant to corrosion because zinc (Zn) provides sacrificial corrosionprotection to the steel sheets.

A hot-dip galvanized steel sheet disclosed in Japanese PatentApplication Laid-open Publication No. 2006-022395 is an example of suchsteel sheets resistant to corrosion. If a steel sheet is subjected to ahot-dip galvanizing process and then an HPF process, an alloy having azinc content of 70% or greater is formed on the steel sheet, and thusthe corrosion resistance of the steel sheet is improved.

However, if a galvanized steel sheet such as a hot-dip galvanized steelsheet is heated in high-temperature air, zinc oxides are formed on thegalvanized steel sheet, and after a forming process, such zinc oxidesfunction as a resistor hindering the flow of current during a weldingprocess such as a spot welding process, thereby worsening theweldability of the galvanized steel sheet. Due to this reason,additional processes may be performed after a forming process so as toremove zinc oxides from such a galvanized steel sheet and thus toimprove the weldability of the galvanized steel sheet during a spotwelding process. However, this increases manufacturing costs.

A method of preventing the formation of oxides on a steel sheet after anHPF process is disclosed in U.S. Pat. No. 6,296,805. According to thedisclosed method, a steel sheet is plated with aluminum (Al) so as to beused as a steel sheet for HPF. If a steel sheet is plated with aluminum(Al) having a high degree of heat resistance and is then subjected to anHPF process, a product formed by the HPF process does not have surfaceoxides or has very low amounts of surface oxides, and thus the spotweldability of the product is very high. However, unlike zinc (Zn),aluminum provides very insufficient sacrificial corrosion protection toa base steel sheet, and thus if the iron surface of the base steel sheetis exposed, the base steel sheet may markedly corrode.

DISCLOSURE Technical Problem

To solve above-mentioned problems, an aspect of the present disclosuremay provide a plated steel sheet for hot press forming (HPF). The platedsteel sheet has a high degree of spot weldability because the thicknessof surface oxides of the plated steel sheet is small even after an HPFprocess. In addition, the plated steel sheet is resistant to corrosionbecause an alloy layer formed on an iron surface of the plated steelsheet is electrochemically less noble than iron (Fe) of the plated steelsheet, and thus provides sacrificial corrosion protection to iron (Fe)of the plated steel sheet. Aspects of the present disclosure may alsoprovide a hot-press formed member manufactured using the plated steelsheet and methods of manufacturing the plated steel sheet and thehot-press formed member.

Aspects of the present disclosure are not limited to the above-mentionedaspects. The above-mentioned aspects and other aspects of the presentdisclosure will be clearly understood by those skilled in the artthrough the following description.

Technical Solution

According to an aspect of the present disclosure, a plated steel sheetmay include: a base steel sheet; and a composite plating layer formed onat least one side of the base steel sheet, the composite plating layerincluding a Mn-based plating layer and an Al-based plating layeralternately formed with the Al-based plating layer being an uppermostlayer, wherein the composite plating layer may have a total thickness of5 μm to 30 μm, and the Mn-based plating layer may have a thicknessaccounting for 5% to 60% of the total thickness of the composite platinglayer.

The composite plating layer may have a total thickness of 5 μm to 25 μm,and the Mn-based plating layer may have a thickness accounting for 20%to 50% of the total thickness of the composite plating layer.

The Mn-based plating layer may include at least one selected from thegroup consisting of chromium (Cr), zinc (Zn), beryllium (Be), magnesium(Mg), calcium (Ca), and titanium (Ti) in an amount of 20 wt % or less(excluding 0 wt %).

The Al-based plating layer may include at least one selected from thegroup consisting of zinc (Zn), silicon (Si), magnesium (Mg), andmanganese (Mn) in an amount of 20 wt % or less (excluding 0 wt %).

The plated steel sheet may be for hot press foLming (HPF).

According to another aspect of the present disclosure, a hot-pressformed member formed by performing an HPF process on the plated steelsheet may include: a base steel sheet; and an alloy layer formed on atleast one side of the base steel sheet.

The alloy layer may include a plurality of alloys. For example, thealloy layer may include at least two alloys selected from the groupconsisting of an Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, anFe-Al-based alloy, and an Al-Mn-based alloy.

The alloy layer may further include at least one alloy selected from thegroup consisting of a Zn-Mn-based alloy, a Zn-Fe-based alloy, anFe-Mn-Al-Mg-based alloy, and an Fe-Mn-Al-Si-based alloy.

The alloy layer may include an uppermost layer formed of anAl-containing alloy.

For example, the alloy layer may include an uppermost layer formed of anFe-Al-Mn-based alloy, an Fe-Al-based alloy, or an Al-Mn-based alloy.

For example, the alloy layer may include an uppermost layer formed of anFe-Al-Mn-based alloy, an Fe-Al-based alloy, or an Al-Mn-based alloy, anFe-Mn-Al-Mg-based alloy, or an Fe-Mn-Al-Si-based alloy.

Surface oxides formed on an outermost surface of the alloy layer mayhave a thickness of 2 μm or less.

According to another aspect of the present disclosure, a method ofmanufacturing a plated steel sheet may include: preparing a base steelsheet; and forming a composite plating layer on at least one side of thebase steel sheet by alternately forming a Mn-based plating layer and anAl-based plating layer with the Al-based plating layer being anuppermost layer, wherein the composite plating layer may have a totalthickness of 5 μm to 30 μm, and the Mn-based plating layer may have athickness accounting for 5% to 60% of the total thickness of thecomposite plating layer.

The Mn-based plating layer may be formed by a dry plating method.

According to another aspect of the present disclosure, a method ofmanufacturing a hot-press formed member may include: heating a platedsteel sheet to a temperature range of an Ac₃ transformation point to1000° C. at an average heating rate of 3° C./s to 200° C./s; hot formingthe heated plated steel sheet within the temperature range; and coolingthe hot-formed plated steel sheet.

After the heating of the plated steel sheet, the method may furtherinclude maintaining the heated plated steel sheet within the temperaturerange for 240 seconds.

The above-described aspects of the present disclosure do not include allaspects or features of the present disclosure. Other aspects orfeatures, and effects of the present disclosure will be clearlyunderstood from the following descriptions of exemplary embodiments.

Advantageous Effects

According to the present disclosure, since the uppermost layer of thecomposite plating layer of the plated steel sheet is an Al-based platinglayer, the hot-press formed member manufactured by performing a hotpress forming (HPF) process on the plated steel sheet has a thin surfaceoxide layer, and thus a high degree of spot weldability.

In addition, according to the present disclosure, the hot-press formedmember formed by performing an HPF process on the plated steel sheet hasalloys such as an Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, anFe-Al-based alloy, or an Al-Mn-based alloy because alloying occursbetween the composite plating layer and iron (Fe) of the plated steelsheet during heating, and since the alloys are less noble than iron (Fe)of the plated steel sheet, the alloys provide sacrificial corrosionprotection to iron (Fe) of the plated steel sheet. Therefore, thehot-press formed member has a high degree of corrosion resistance.

DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are schematic cross-sectional views illustratingexemplary plated steel sheets for hot press forming (HPF) includingcomposite plating layers according to an exemplary embodiment of thepresent disclosure.

FIGS. 2A to 2D are schematic cross-sectional views illustratingexemplary hot-press formed members including alloy layers according toan exemplary embodiment of the present disclosure.

FIG. 3 is a schematic cross-sectional view illustrating a hot-pressformed member including an oxide layer according to an exemplaryembodiment of the present disclosure.

BEST MODE FOR INVENTION

Exemplary embodiments of the present disclosure will now be described indetail with reference to the accompanying drawings. The disclosure may,however, be exemplified in many different forms and should not beconstrued as being limited to the specific embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the disclosureto those skilled in the art. In the drawings, the shapes and dimensionsof elements may be exaggerated for clarity.

1. Plated Steel Sheet

Since aluminum plated steel sheets have heat resistance, very thinoxides layers are formed on aluminum plated steel sheets after a heatingand forming process is performed on the aluminum plated steel sheets.Thus, products formed of aluminum plated steel sheets have a high degreeof spot weldability. However, after a forming process is performed onaluminum plated steel sheets, alloy layers formed on iron surfaces ofthe steel sheets have a base electrochemical potential insufficient forprotecting the steel sheets, and thus do not provide sacrificialcorrosion protection to the steel sheets. Therefore, it is required toplate a steel sheet for hot press forming (HPF) with a heat-resistantplating layer in which an alloy having a sufficient base electrochemicalpotential for protecting iron (Fe) of the steel sheet is formed after aheating and forming process.

The inventors have conducted much research to achieve theabove-mentioned objects and found that if a composite plating layerhaving a total thickness of 5 μm to 30 μm is formed on a base steelsheet by alternately forming a Mn-based plating layer and an Al-basedplating layer in such a manner that the uppermost layer of the compositeplating layer is the Al-based plating layer and the Mn-based platinglayer accounts for 60% or less of the total thickness of the compositelayer, alloying occurs between the composite plating layer and iron (Fe)of the base steel sheet in a later heating process, and thus alloys suchas an Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, an Fe-Al-based alloy,or an Al-Mn-based alloy are formed. Then, when a press forming processis performed on the steel sheet plated with the composite plating layer,very small amounts of surface oxides are formed on the plated steelsheet, and thus the plated steel sheet has a high degree of spotweldability and a high degree of corrosion resistance. Based on thisknowledge, the inventors have invented the present invention.

In more detail, according to an exemplary embodiment of the presentdisclosure, a plated steel sheet for HPF includes: a base steel sheet;and a composite plating layer formed on at least one side of the basesteel sheet by alternately forming a Mn-based plating layer and anAl-based plating layer with the uppermost layer being the Al-basedplating layer, wherein the composite plating layer has a total thicknessof 5 μm to 30 μm, and the Mn-based plating layer accounts for 5% to 60%of the total thickness of the composite plating layer.

The base steel sheet may be a general carbon steel sheet. For example,the base steel sheet may include: carbon (C): 0.1 wt % to 0.4 wt %,silicon (Si): 0.05 wt % to 1.5 wt %, manganese (Mn): 0.5 wt % to 3.0 wt%, and a balance of iron (Fe) and inevitable impurities. However, thebase steel sheet is not limited thereto.

Carbon (C): 0.1 wt % to 0.4 wt %

Carbon (C) is a very effective element in increasing the strength of thebase steel sheet. However, if the content of carbon (C) in the basesteel sheet is too high, the weldability and low-temperature toughnessof the steel sheet are decreased. If the content of carbon (C) in thebase steel sheet is less than 0.1 wt %, even though an HPF process isperformed in an austenite single phase, it may be difficult to obtain anintended degree of strength. Conversely, if the content of carbon (C) inthe base steel sheet is greater than 0.4 wt %, the weldability andlow-temperature toughness of the base steel sheet may deteriorate, andthe strength of the base steel sheet may excessively increase to causedisadvantages in manufacturing processes such as difficulty intransferring the base steel sheet during an annealing process and aplating process. Therefore, it may be preferable that the content ofcarbon (C) be within a range of 0.1 wt % to 0.4 wt %.

Silicon (Si): 0.05 wt % to 1.5 wt %

Silicon (Si) is an element added to remove oxygen and improve strengthby solid-solution strengthening. In the exemplary embodiment, to obtainthese effects, it may be preferable that the content of silicon (Si) bewithin a range of 0.05 wt % or greater. However, if the content ofsilicon (Si) is greater than 1.5 wt %, it may be difficult to perform apickling process on a hot-rolled steel sheet, and thus surface defectssuch as scales may remain due to poor pickling or oxides not removedthrough a pickling process.

Manganese (Mn): 0.5 wt % to 3.0 wt %

Manganese (Mn) is an effective element in increasing strength bysolid-solution strengthening and retarding transformation from austeniteto ferrite. In the exemplary embodiment, to obtain these effects, it maybe preferable that the content of manganese (Mn) be within a range of0.5 wt % or greater. However, if the content of manganese (Mn) in thebase steel sheet is greater than 3.0 wt %, properties of the base steelsheet such as weldability and hot-rolling properties may deteriorate.

In the exemplary embodiment of the present disclosure, the othercomponent of the base steel sheet is iron (Fe). However, impurities ofraw materials or manufacturing environments may be inevitably includedin the base steel sheet, and such impurities may not be removed from thebase steel sheet. Such impurities are well-known to those of ordinaryskill in manufacturing industries, and thus descriptions thereof willnot be given in the present disclosure.

If the base steel sheet of the exemplary embodiment includes theabove-mentioned alloying elements within the above-mentioned contentranges, intended effects may be sufficiently obtained. However, the basesteel sheet of the exemplary embodiment may further include at least oneselected from the group consisting of nitrogen (N): 0.001 wt % 0.02 wt%, boron (B): 0.0001 wt % to 0.01 wt %, titanium (Ti): 0.001 wt % to 0.1wt %, niobium (Nb): 0.001 wt % to 0.1 wt %, vanadium (V): 0.001 wt % to0.01 wt %, chromium (Cr): 0.001 wt % to 1.0 wt %, molybdenum (Mo): 0.001wt % to 1.0 wt %, antimony (Sb): 0.001 wt % to 0.1 wt %, and tungsten(W): 0.001 wt % to 0.3 wt %. In this case, properties of the base steelsheet such as strength, toughness, or weldability may be furtherimproved.

Nitrogen (N): 0.001 wt % to 0.02 wt %

Nitrogen (N) combines with aluminum (Al) in austenite grains duringsolidification, thereby leading to the precipitation of fine nitridesand facilitating twining. In this manner, nitrogen (N) improves thestrength and ductility of the base steel sheet during a forming process.However, if the content of nitrogen (N) in the base steel sheetincreases, the precipitation of nitrides may excessively occur, and thusthe hot-rolling properties and elongation of the base steel sheet maydeteriorate or decrease. Thus, it may be required to adjust the contentof nitrogen (N). If the content of nitrogen (N) is less than 0.001 wt %,excessively high manufacturing costs may be incurred so as to adjust thecontent of nitrogen (N) during a steel making process. Conversely, ifthe content of nitrogen (N) is greater than 0.02 wt %, the precipitationof nitrides may excessively occur, thereby worsening the hot-rollingproperties and elongation of the base steel sheet and causing theformation of cracks.

Boron (B): 0.0001 wt % to 0.01 wt %

Boron (B) retards transformation from austenite to ferrite. In theexemplary embodiment, to obtain this effect, it may be preferable thatthe content of boron (B) be within a range of 0.0001 wt % or greater.However, if the content of boron (B) is greater than 0.01 wt %, thehot-rolling properties of the base steel sheet may deteriorate.

Each of titanium (Ti), niobium (Nb), and vanadium (V): 0.001 wt % to 0.1wt %

Titanium (Ti), niobium (Nb), and vanadium (V) are effective elements inincreasing the strength of the base steel sheet, reducing the grain sizeof the base steel sheet, and improving the heat treatability of the basesteel sheet. In the exemplary embodiment, to obtain these effects, itmay be preferable that the content of each of titanium (Ti), niobium(Nb), and vanadium (V) be within a range of 0.001 wt % or greater.However, if the content of each of titanium (Ti), niobium (Nb), andvanadium (V) is greater than 0.1 wt %, manufacturing costs increase, andit is difficult to obtain intended degrees of strength and yieldstrength because of excessive formation of carbides and nitrides.

Each of chromium (Cr) and molybdenum (Mo): 0.001 wt % to 1.0 wt %

Chromium (Cr) and molybdenum (Mo) are effective in increasing thehardenability and toughness of the base steel sheet. Thus, chromium (Cr)and molybdenum (Mo) may be usefully added to the base steel sheet sothat the base steel sheet may absorb a large amount of impact energy.However, if the content of each of chromium (Cr) and molybdenum (Mo) isless than 0.001 wt %, the above-described effects may not besufficiently obtained, and if the content of each of chromium (Cr) andmolybdenum (Mo) is greater than 1.0%, the above-described effects do notfurther increase and manufacturing costs increase.

Antimony (Sb): 0.001 wt % to 0.1 wt %

Antimony (Sb) leads to the formation of uniform scales by preventingselective oxidation of grain boundaries during a hot rolling process,and improves pickling properties of hot-rolled steel sheets. In theexemplary embodiment, to obtain these effects, it may be preferable thatthe content of antimony (Sb) be within a range of 0.001 wt % or greater.If the content of antimony (Sb) is greater than 0.1 wt %, theabove-described effects do not further increase, and manufacturing costsincrease and embrittlement occurs during a hot rolling process.

Tungsten (W): 0.001 wt % to 0.3 wt %

Tungsten (W) is an element improving the hardenability of the base steelsheet. In addition, tungsten-containing precipitates have an effect ofguaranteeing strength. In the exemplary embodiment, to obtain theseeffects, it may be preferable that the content of tungsten (W) be withina range of 0.001 wt % or greater. If the content of tungsten (W) isgreater than 0.3 wt %, the above-described effects do not furtherincrease, and manufacturing costs increase.

Next, according to an exemplary embodiment of the present disclosure,the composite plating layer is formed by alternately forming a Mn-basedplating layer and an Al-based plating layer with the uppermost layerbeing the Al-based plating layer. The composite plating layer has atotal thickness of 5 μm to 30 μm, and the thickness of the Mn-basedplating layer is preferably 5% to 60% of the total thickness of thecomposite plating layer.

Manganese (Mn) is electrochemically less noble than iron (Fe). Ifmanganese (Mn) combines with aluminum (Al) or iron (Fe), alloys such asan Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, or an Al-Mn-based alloyare formed, and the alloys are electrochemically less noble thanaluminum (Al) or an Al-Fe-based alloy. Therefore, if the content ofmanganese (Mn) in the composite plating layer increases, after a heatingand forming process, the potential of an alloy layer becomes less noble,and thus the corrosion of iron (Fe) of the base steel sheet may be moreeffectively prevented by sacrificial corrosion protection. However, ifthe thickness of the Mn-based plating layer accounts for a large portionof the total thickness of the composite plating layer, even thoughsacrificial corrosion protection is reliably provided to iron (Fe) ofthe base steel sheet, manganese (Mn) may diffuse to a surface layer, andsome of the diffused manganese (Mn) may form manganese oxides. In thiscase, spot weldability may decrease because such surface manganeseoxides have electric resistance higher than those of metals. Inaddition, if the thickness of the Mn-based plating layer accounts for atoo small portion of the total thickness of the composite plating layer,alloys such as an Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, or anAl-Mn-based alloy may not be formed or may be formed in too smallamounts by alloying between manganese (Mn) and aluminum (Al) or iron(Fe). In this case, sacrificial corrosion protection may be hardlyprovided by an alloy layer, and thus the corrosion resistance of theplated steel sheet may be insufficient. Therefore, according to theexemplary embodiment, preferably, the thickness of the Mn-based platinglayer may be 5% to 60% of the total thickness of the composite platinglayer, for example, about 10% to about 55%, or about 20% to about 50% ofthe total thickness of the composite plating layer.

If the composite plating layer is too thin, sufficient corrosionresistance is not guaranteed, and if the composite plating layer is toothick, manufacturing costs increase even though corrosion resistance issufficient. Therefore, it may be preferable that the thickness of thecomposite plating layer be within the range of 5 μm to 30 μm, forexample, about 5 μm to about 25 μm. Herein, the thickness of thecomposite plating layer refers to a value measured on one side of theplated steel sheet.

The composite plating layer may include two plating layers: one Mn-basedplating layer and one Al-based plating layer, or may include at leastthree plating layers. In any case, the uppermost layer of the compositeplating layer may be an Al-based plating layer.

For example, the composite plating layer may include two layers in theorder of a Mn-based plating layer and an Al-based plating layer asillustrated in FIG. 1A; three layers in the order of an Al-based platinglayer, a Mn-based plating layer, and an Al-based plating layer asillustrated in FIG. 1B; or four layers in the order of a Mn-basedplating layer, an Al-based plating layer, a Mn-based plating layer, andan Al-based plating layer as illustrated in FIG. 1C.

If the uppermost layer of the composite plating layer is a Mn-basedplating layer, oxides containing manganese (Mn) as the main componentmay be formed in a surface layer of the plated steel sheet after an HPFprocess, and thus the spot weldability of the plated steel sheet may beworsened.

In addition, preferably, the Mn-based plating layer may include:manganese (Mn) in an amount of 80 wt % or greater; and at least oneselected from the group consisting of chromium (Cr), zinc (Zn),beryllium (Be), magnesium (Mg), calcium (Ca), and titanium (Ti) in anamount of 20 wt % or less (excluding 0 wt %), which areelectrochemically less noble than iron (Fe). Even in this case, a highdegree of weldability and a high degree of corrosion resistance may beguaranteed after an HPF process as intended in the exemplary embodimentof the present disclosure.

In addition, preferably, the Al-based plating layer may include:aluminum (Al) in an amount of 80 wt % or greater; and at least oneselected from the group consisting of zinc (Zn), silicon (Si), magnesium(Mg), and manganese (Mn) in an amount of 20 wt % or less (excluding 0 wt%) so as to control the hardness and plating bath toxicity of theAl-based plating layer. Even in this case, a high degree of weldabilityand a high degree of corrosion resistance may be guaranteed after an HPFprocess as intended in the exemplary embodiment of the presentdisclosure.

In an HPF process, when the plated steel sheet is heated to atemperature range of an Ac₃ transformation point to 1000° C. so as tohot-press the plated steel sheet, alloying may occur between thecomposite plating layer and iron (Fe) of the plated steel sheet, andthus at least two alloys selected from the group consisting of anFe-Mn-based alloy, an Fe-Al-Mn-based alloy, an Fe-Al-based alloy, and anAl-Mn-based alloy may be formed. In this case, very small amounts ofsurface oxides may be formed after the HPF process, and thus the platedsteel sheet may have a high degree of spot weldability and a high degreeof corrosion resistance.

In an HPF process, when the plated steel sheet is heated to atemperature range of an Ac₃ transformation point to 1000° C. so as tohot-press the plated steel sheet, additional elements may undergoalloying, and thus at least one alloy selected from the group consistingof a Zn-Mn-based alloy, a Zn-Fe-based alloy, an Fe-Mn-Al-Mg-based alloy,and an Fe-Mn-Al-Si-based alloy may be additionally formed. The at leastone additional alloy is not limited to the listed alloys. That is, theat least one additional alloy may be varied according to elementsadditionally added to the composite plating layer. Even in this case,very small amounts of surface oxides may be formed after the HPFprocess, and thus the plated steel sheet may have a high degree of spotweldability and a high degree of corrosion resistance.

2. Method of Manufacturing Plated Steel Sheet

Hereinafter, a method of manufacturing a plated steel sheet will bedescribed according to an exemplary embodiment of the presentdisclosure.

According to the exemplary embodiment, the method of manufacturing aplated steel sheet includes: preparing a base steel sheet; and forming acomposite plating layer on at least one side of the base steel sheet byalternately forming a Mn-based plating layer and an Al-based platinglayer with the uppermost plating layer being the Al-based plating layer,wherein the composite plating layer has a total thickness of 5 μm to 30μm, and the thickness of the Mn-based plating layer accounts for 5% to60% of the total thickness of the composite plating layer.

First, the base steel sheet may be prepared by any manufacturing methodas long as the base steel sheet includes the above-described elements.For example, the base steel sheet may be prepared by a manufacturingmethod known in the related art. For example, a commercially availablesteel sheet including the above-described elements may be prepared asthe base steel sheet.

Next, the composite plating layer is formed on at least one side of thebase steel sheet by alternately forming a Mn-based plating layer and anAl-based plating layer with the uppermost layer being the Al-basedplating layer. At this time, preferably, the composite plating layer mayhave a total thickness of 5 μm to 30 μm, and the thickness of theMn-based plating layer may be 5% to 60% of the total thickness of thecomposite plating layer.

The composite plating layer may be formed by a plating method known inthe related art. For example, the Mn-based plating layer may be formedby a deposition method, and the Al-based plating layer may be formed bya hot dipping method or a deposition method. In the exemplary embodimentof the present disclosure, a dry plating method may be used for theefficiency of a plating process, and ease in performing the platingprocess and adjusting the amount of plating.

3. Hot-Press Formed Member

Hereinafter, a hot-press formed member manufactured using the platedsteel sheet will be described according to an exemplary embodiment ofthe present disclosure.

The hot-press formed member of the exemplary embodiment may be obtainedby performing an HPF process on the above-described plated steel sheet,and the hot-press formed member may include a base steel sheet and analloy layer formed on at least one side of the base steel sheet. Ageneral steel sheet having the above-described elements may be used asthe base steel sheet.

As described above, the alloy layer is formed by alloying between iron(Fe) and the composite plating layer of the plated steel sheet, and thealloy layer includes a plurality of alloys. In detail, the alloy layermay include at least two alloys selected from the group consisting of anFe-Mn-based alloy, an Fe-Al-Mn-based alloy, an Fe-Al-based alloy, and anAl-Mn-based alloy. For example, the alloy layer may include: anFe-Al-based alloy and an Fe-Al-Mn-based alloy as illustrated in FIG. 2A;an Fe-Al-based alloy, an Al-Mn-based alloy, and an Fe-Al-Mn-based alloyas illustrated in FIG. 2B; an Fe-Al-Mn-based alloy, an Al-Mn-basedalloy, an Fe-Al-Mn-based alloy, and an Fe-Mn-based alloy as illustratedin FIG. 2C; or an Fe-Al-based alloy, an Al-Mn-based alloy, anFe-Al-Mn-based alloy, and an Fe-Mn-based alloy as illustrated in FIG.2D. However, the alloy layer of the exemplary embodiment is not limitedto the examples. That is, the alloy layer of the exemplary embodiment isnot limited to the above-described combinations of alloys. In addition,the alloy layer of the exemplary embodiment is not limited to theabove-described order of alloys.

As described above, when alloying occurs between the composite platinglayer and iron (Fe) of the plated steel sheet, additional elements mayalso undergo alloying, and thus at least one alloy selected from thegroup consisting of a Zn-Mn-based alloy, a Zn-Fe-based alloy, anFe-Mn-Al-Mg-based alloy, and an Fe-Mn-Al-Si-based alloy may beadditionally formed. The at least one additional alloy is not limited tothe listed alloys. That is, the at least one additional alloy may bevaried according to elements additionally added to the composite platinglayer of the plated steel sheet.

As illustrated in FIG. 3, oxides may be formed on the outermost surfaceof the alloy layer of the hot-press formed member of the exemplaryembodiment. In this case, the oxides may be formed on the outermostsurface of the alloy layer to a thickness of 2 μm or less, preferably 1μm or less. If the surface oxides are formed to a small thickness asdescribed above, the hot-press formed member may have a high degree ofspot weldability.

In addition, the uppermost layer of the alloy layer may be formed of analloy including aluminum (Al). That is, according to the exemplaryembodiment, the alloy layer may include at least two alloys, and analloy including aluminum (Al) may exist in the uppermost layer of thealloy layer, thereby leading to the formation of a thin surface oxidelayer.

The alloy including aluminum (Al) may be an Fe-Al-Mn-based alloy, anFe-Al-based alloy, or an Al-Mn-based alloy. Alternatively, owing toadditional elements, the alloy including aluminum (Al) may be anFe-Al-Mn-based alloy, an Fe-Al-based alloy, an Al-Mn-based alloy, anFe-Mn-Al-Mg-based alloy, or an Fe-Mn-Al-Si-based alloy. However, thealloy including aluminum (Al) is not limited thereto. That is, the alloyincluding aluminum (Al) may be varied according to elements included inthe composite plating layer of the plated steel sheet.

4. Method of Manufacturing Hot-Press Formed Member

Hereinafter, a method of manufacturing a hot-press formed member will bedescribed according to an embodiment of the present disclosure.

According to the exemplary embodiment, the method of manufacturing ahot-press formed member includes: heating the above-described platedsteel sheet to a temperature range of an Ac3 transformation point to1000° C. at an average heating rate of 3° C./s to 200° C./s; hot formingthe heated plated steel sheet within the temperature range; and coolingthe hot-formed plated steel sheet.

The heating process may be performed by any method usually used in therelated art such as an electric furnace method, a gas furnace method, aflame heating method, an electric heating method, a high frequencyheating method, or an induction heating method. However, the heatingprocess is not limited thereto.

In the heating process, the plated steel sheet may be heated to atemperature range of about the Ac₃ transformation point(ferrite-to-austenite transformation temperature) to about 1000° C.,preferably, to a temperature range of about the Ac₃ transformation point(ferrite-to-austenite transformation temperature) to about 950° C. Whenthe plated steel sheet is heated to the above-mentioned heatingtemperature range, alloying occurs between iron (Fe) and the compositeplating layer of the plated steel sheet, and thus alloys such as anFe-Mn-based alloy, an Fe-Al-Mn-based alloy, an Fe-Al-based alloy, or anAl-Mn-based alloy are formed.

Furthermore, the heating process may be performed at an average heatingrate of about 3° C./s to about 200° C./s, and more preferably about 5°C./s to about 150° C./s. If the heating rate is within theabove-mentioned range, high corrosion resistance, high productivity, andthe formation of a thin surface oxide layer may be guaranteed. Forexample, if the heating rate is lower than the above-mentioned range, itwill take much time to reach an intended temperature, and thus surfaceoxides may be formed to a thickness of 1 μm or greater due to a longheating time. In addition, the diffusion of iron (Fe) of the platedsteel sheet into the composite plating layer may increase, and thus thecontent of iron (Fe) in an alloy layer may increase, thereby loweringcorrosion resistance and productivity.

According to the exemplary embodiment, after the heating process, thetemperature of the heated plated steel sheet may be maintained withinthe heating temperature range of the heating process for a predeterminedtime period so as to obtain intended material qualities. In this case,if the predetermined time period is too long, a time period during whichlower manganese (Mn) or iron (Fe) diffuses to a surface layer and formsoxides by combining with oxygen is also increased. As a result, a thickoxide layer may be formed, and spot weldability may be lowered.Therefore, preferably, the predetermined time period may be set to bewithin the range of 240 seconds or shorter, for example, about 10seconds to about 200 seconds, so as to adjust the thickness of an oxidelayer to be 1 μm or less.

The hot forming process may be performed by any method usually used inthe related art. For example, in a state in which the temperature of theheated plated steel sheet is maintained within the heating temperaturerange, the heated plated steel sheet may be hot formed into an intendedshape by using a press machine. However, the hot forming process is notlimited thereto.

In the cooling process, the hot-formed plated steel sheet may preferablybe cooled to 100° C. at a cooling rate of 10° C./s or higher. If thecooling rate is lower than 10° C./s, austenite-to-ferrite transformationmay increase, and thus the strength of the hot-formed plated steel sheetmay decrease after the cooling process.

[Mode for Invention]

Hereinafter, the present disclosure will be described more specificallyaccording to examples.

First, general cold-rolled steel sheets for hot press forming (HPF)having a thickness of 1.5 mm were prepared as base steel sheets. Thebase steel sheets included carbon (C): 0.22 wt %, silicon (Si): 0.24 wt%, manganese (Mn): 1.56 wt %, phosphorus (P): 0.012 wt %, boron (B):0.0028 wt %, chromium (Cr): 0.01 wt %, titanium (Ti): 0.03 wt %, and abalance of iron (Fe) and inevitable impurities.

Next, plated steel sheets were manufactured by plating the base steelsheets with manganese (Mn) by a deposition plating method according toplating thicknesses illustrated in Table 1 below. At that time, analloying element such as zinc (Zn) or magnesium (Mg) was depositedtogether with manganese (Mn) on some of the base steel sheets. Next, theplated steel sheets were further plated with aluminum (Al) by adeposition plating method. At that time, an alloying element such assilicon (Si) or magnesium (Mg) was deposited together with aluminum (Al)on some of the plated steel sheets. In addition, comparative platedsteel sheets were manufactured by plating the base steel sheets withaluminum (Al), manganese (Mn), or zinc (Zn). The amount of plating ofthe plated steel sheets manufactured as described above was analyzed bydissolving plating layers and converting the dissolved amounts intothicknesses, and the total thickness of the plating layers of each ofthe plated steel sheets was calculated as illustrated in Table 1 below.

TABLE 1 First plating Second plating Total layer layer plating PlatingPlating layer Plating thickness thickness thickness layer PlatingPlating (one Plating (one (one side, thickness No. layers type side, μm)type side, μm) μm) ratio (%) IS1 Two Mn 100% 1.5 Al 100% 5.1 6.6 22.7layers IS2 Two Mn 100% 3.3 Al 100% 8.9 12.2 27 layers IS3 Two Mn 100%6.4 Al 100% 13.9 20.3 31.5 layers IS4 Two Mn 100% 6.2 Al 100% 15.1 21.329.1 layers IS5 Two Mn 100% 6.8 Al 100% 8.1 14.9 45.6 layers IS6 Two Mn100% 9.5 Al 100% 10.1 19.6 48.5 layers IS7 Two Mn 95% 2.2 Al 100% 6.99.1 24.2 layers Zn 5% IS8 Two Mn 81% 4.8 Al 100% 12.4 17.2 27.9 layersZn 19% IS9 Two Mn 100% 5.1 Al 92%, 12.3 17.4 29.3 layers Si 8% IS10 TwoMn 100% 5.2 Al 88%, 12.5 17.7 29.4 layers Si 8%, Mg 4% CS1 Single Al100% 15 — — 15 100 layer CS2 Single Mn 100% 13.8 — — 13.8 100 layer CS3Single Zn 9.5 — — 9.5 100 layer 99.4% Al 0.6% CS4 Two Mn 100% 13.6 Al100% 1.4 15 90.7 layers CS5 Two Mn 100% 13.5 Mn 100% 2.4 15.9 84.9layers CS6 Two Mn 100% 1.2 Al 100% 10.1 11.3 10.6 layers CS7 Two Mn 100%5.2 Al 100% 7.1 12.3 42.3 layers CS8 Two Mn 100% 0.6 Al 100% 15.5 16.13.73 layers IS: Inventive Steel, CS: Comparative Steel

In Table 1, the plating layer thickness ratio was calculated by theexpression: (first plating layer thickness/total plating layerthickness)×100.

Next, an HPF process was performed on each of the plated steel sheetsunder the conditions illustrated in Table 2 below so as to manufactureformed products, and properties of the formed products were evaluated asillustrated in Table 2 below. At that time, the corrosion resistance ofthe formed products was evaluated by performing a salt solution test(SST) on the formed products for 1200 hours and measuring the maximumdepth of corrosion in iron (Fe) of the base steel sheets of the formedproducts.

TABLE 2 Corrosion Surface resistance Heating conditions Cooling oxide(SST, HT AHR MTP rate Alloy (excluding thickness corrosion No. (° C.) (°C./sec) (sec) (° C./sec) oxides) (μm) depth, mm) IE 1 900 10 150 30Fe—Al, Fe—Mn—Al 0.2 0.18 2 880 6 100 30 Fe—Al, Al—Mn, 0.21 0.11 Fe—Mn—Al3 880 120 150 25 Fe—Al, Fe—Mn, 0.23 <0.1 Al—Mn, Fe—Mn—Al 4 930 30 30 60Fe—Al, Fe—Mn, 0.17 <0.1 Al—Mn, Fe—Mn—Al 5 900 10 200 50 Fe—Al, Fe—Mn,0.45 <0.1 Al—Mn, Fe—Mn—Al 6 900 10 100 30 Fe—Al, Fe—Mn, 0.37 <0.1 Al—Mn,Fe—Mn—Al 7 900 10 150 30 Fe—Al, Al—Mn, 0.68 0.14 Fe—Mn—Al, Fe—Mn—Al—Mg 8900 10 150 20 Fe—Al, Al—Mn, 0.25 <0.1 Fe—Mn—Al, Zn—Mn 9 900 10 150 20Fe—Al, Al—Mn, 0.20 <0.1 Fe—Mn—Al, Fe—Mn—Al—Si 10 900 10 150 20 Fe—Al,Al—Mn, 0.73 <0.1 Fe—Mn—Al, Fe—Mn—Al—Si CE 1 930 10 150 30 Fe—Al 0.140.51 2 900 10 150 30 Fe—Mn >2 0.22 3 930 10 150 30 Fe—Zn >2 0.33 4 90010 150 30 Fe—Mn, Al—Mn >2 0.20 5 900 10 150 30 Fe—Al, Al—Mn, >2 0.45Fe—Mn—Al 6 900 1 200 30 Fe—Al, Fe—Mn, 1.1 0.22 Al—Mn, Fe—Mn—Al 7 900 6400 30 Fe—Al, Fe—Mn, 1.4 <0.1 Al—Mn, Fe—Mn—Al 8 900 6 150 30 Fe—Al,Fe—Mn—Al 0.21 0.46 IE: Inventive Example, CE: Comparative Example, HT:Heating Temperature, AHR: Average heating rate, MTP: Maintaining TimePeriod

As shown in Tables 1 and 2, each of the hot-press formed members (formedproducts) of Inventive Examples 1 to 6 included at least two of anFe-Mn-based alloy, an Fe-Al-Mn-based alloy, an Fe-Al-based alloy, and anAl-Mn-based alloy on an iron surface of the base steel sheet thereof,and the uppermost layer of each of the hot-press formed members ofInventive Examples 1 to 6 included an Al-based alloy. Thus, thethickness of a surface oxide layer of each of the hot-press formedmembers of Inventive Examples 1 to 6 was less than 1 μm. After the SST,the depth of corrosion in iron (Fe) of the base steel sheet of each ofthe hot-press formed members of Inventive Examples 1 to 6 was 0.18 mm orless. That is, the hot-press formed members of Inventive Examples 1 to 6had a high degree of corrosion resistance.

In addition, each of the hot-press formed members of Inventive Examples7 and 8 included at least two of an Fe-Mn-based alloy, an Fe-Al-Mn-basedalloy, an Fe-Al-based alloy, an Al-Mn-based alloy, an Fe-Mn-Al-Mg-basedalloy, and a Zn-Mn-based alloy on an iron surface of the base steelsheet thereof, and the uppermost layer of each of the hot-press formedmembers of Inventive Examples 7 and 8 included an Al-based alloy. Thus,the thickness of a surface oxide layer of each of the hot-press formedmembers of Inventive Examples 7 and 8 was less than 1 μm. After the SST,the depth of corrosion in iron (Fe) of the base steel sheet of each ofthe hot-press formed members of Inventive Examples 7 and 8 was 0.14 mmor less. That is, the hot-press formed members of Inventive Examples 7and 8 had a high degree of corrosion resistance.

In addition, each of the hot-press formed members of Inventive Examples9 and 10 included at least two of an Fe-Al-based alloy, an Al-Mn-basedalloy, an Fe-Al-Mn-based alloy, and an Fe-Mn-Al-Si-based alloy on aniron surface of the base steel sheet thereof, and the uppermost layer ofeach of the hot-press formed members of Inventive Examples 9 and 10included an Al-based alloy. Thus, the thickness of a surface oxide layerof each of the hot-press formed members of Inventive Examples 9 and 10was less than 1 μm. After the SST, the depth of corrosion in iron (Fe)of the base steel sheet of each of the hot-press formed members ofInventive Examples 9 and 10 was 0.1 mm or less. That is, the hot-pressformed members of Inventive Examples 9 and 10 had a high degree ofcorrosion resistance.

However, in the case of the hot-press formed member of ComparativeExample 1 having only a single Al plating layer of 15 μm in thickness,an Fe-Al-based alloy having heat resistance was formed on an ironsurface of the hot-press formed member, and thus the thickness of asurface oxide layer was 1 μm or less. However, since the Al platinglayer did not provide sacrificial corrosion protection to iron (Fe) ofthe base steel sheet of the hot-press formed member, the depth ofcorrosion in iron (Fe) of the base steel sheet was 0.51 mm after theSST. That is, the corrosion resistance of the hot-press formed member ofComparative Example 1 was poor.

In the case of the hot-press formed member of Comparative Example 2having only a single Mn plating layer of 13.8 μm in thickness, anFe-Mn-based alloy was formed on an iron surface of the hot-press formedmember, and a thick surface oxide layer having a thickness of greaterthan 2 μm was formed because of oxidation of surface manganese (Mn).However, since the Fe-Mn-based alloy provided sacrificial corrosionprotection to iron (Fe) of the base steel sheet of the hot-press formedmember, the depth of corrosion in iron (Fe) of the base steel sheet was0.22 mm after the SST. That is, the corrosion resistance of thehot-press formed member was somewhat good.

In the case of Comparative Example 3 using a hot-dip galvanized steelsheet, zinc (Zn) was oxidized at a high temperature, and thus a thickoxide layer having a thickness of greater than 2 μm was formed. Inaddition, the depth of corrosion in iron (Fe) of the steel sheet wasrelatively great at 0.33 mm.

In the case of Comparative Example 4, a Mn plating layer was formed as alower plating layer, and an Al plating layer was formed as an upperplating layer according to the present disclosure. However, thethickness of the lower Mn plating layer was greater than the rangeproposed in the present disclosure, that is, greater than 60% of thetotal thickness of the plating layers, and thus some manganese (Mn) ofthe lower Mn plating layer diffused to a surface layer and formed oxidesduring a heat treatment process. As a result, the thickness of a surfaceoxide layer was greater than the range proposed in the presentdisclosure, that is, greater than 2 μm. However, since an alloy layerprovided sacrificial corrosion protection to iron (Fe) of the base steelsheet, the depth of corrosion in iron (Fe) of the base steel sheet wasrelatively small at 0.2 mm.

In the case of Comparative Example 5 in which an Al plating layer wasformed as a lower plating layer and a Mn plating layer was formed as anupper plating layer, manganese (Mn) of the upper Mn plating layer formedsurface oxides during a heat treatment process, and thus the thicknessof a surface oxide layer was greater than 2 μm, that is, greater thanthe range proposed in the present disclosure. In addition, since mostmanganese (Mn) of the upper Mn plating layer was oxidized, the contentof manganese (Mn) in an alloy layer was low, and thus poor sacrificialcorrosion protection was provided to iron (Fe) of the base steel sheet.Thus, the depth of corrosion in iron (Fe) of the base steel sheet was0.45 mm. That is, corrosion resistance was poor.

In the case of Comparative Examples 6 and 7, a Mn plating layer wasformed, and an Al plating layer was formed on the Mn plating layer insuch a manner that the thickness of the Mn plating layer was 60% or lessof the total thickness of the plating layers and the total thickness ofthe plating layers was 5 μm to 30 μm as proposed in the presentdisclosure. However, an average heating rate and a maintaining timeperiod were outside the ranges proposed in the present disclosure. Thatis, due to a long heating time period, an oxide layer thicker than thoseof the inventive examples was formed. However, an alloy layer includedat least two of an Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, anFe-Al-based alloy, and an Al-Mn-based alloy as proposed in the presentdisclosure, and thus sacrificial corrosion protection was provided toiron (Fe) of the base steel sheet. As a result, the depth of corrosionin iron (Fe) of the base steel sheet was satisfactorily 0.22 mm or less.

In the case of Comparative Example 8, a Mn plating layer was formed as alower plating layer and an Al plating layer was formed as an upperplating layer according to the present disclosure. However, thethickness of the lower Mn plating layer was smaller than the rangeproposed in the present disclosure, that is, smaller than 5% of thetotal thickness of the plating layers. Thus, due to the small thicknessof the Mn plating layer, after a heating process, an Fe-Al-based alloywas mainly formed in an alloy layer, and an Fe-Mn-Al-based alloy wasformed in a thin region of the alloy layer. As a result, poor corrosionresistance was observed in a corrosion test due to poor sacrificialcorrosion protection.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

1. A hot-press formed member comprising: a base steel sheet; and analloy layer formed on at least one side of the base steel sheet, whereinthe alloy layer comprises at least two alloys selected from the groupconsisting of an Fe-Mn-based alloy, an Fe-Al-Mn-based alloy, anFe-Al-based alloy, and an Al-Mn-based alloy.
 2. The hot-press formedmember of claim 1, wherein the alloy layer comprises an uppermost layerformed of an Fe-Al-Mn-based alloy, an Fe-Al-based alloy, or anAl-Mn-based alloy.
 3. The hot-press formed member of claim 1, whereinthe alloy layer further comprises at least one alloy selected from thegroup consisting of a Zn-Mn-based alloy, a Zn-Fe-based alloy, anFe-Mn-Al-Mg-based alloy, and an Fe-Mn-Al-Si-based alloy.
 4. Thehot-press formed member of claim 3, wherein the alloy layer comprises anuppermost layer formed of an Fe-Al-Mn-based alloy, an Fe-Al-based alloy,an Al-Mn-based alloy, an Fe-Mn-Al-Mg-based alloy, or anFe-Mn-Al-Si-based alloy.
 5. The hot-press formed member of claim 1,wherein the alloy layer comprises an uppermost layer formed of anAl-containing alloy.
 6. The hot-press formed member of claim 1, whereinsurface oxides formed on an outermost surface of the alloy layer have athickness of 2 μm or less.